The present invention generally relates to DC motor control devices and methods. More particularly, the present invention pertains to devices and methods for controlling a DC motor in which motor speed is calculated using motor field flux and armature voltage parameters.
Conventional DC motor controllers are often used to control the speed and torque of the motor under various operation conditions. Typically this requires the use of a control loop that depends on an accurate determination of motor speed. In many DC motor controllers, motor speed is calculated using measurements of the motor field flux and armature voltage.
Although it is easy to measure the armature voltage in an operating DC motor, motor field flux cannot easily be measured directly. Consequently, many conventional motor control systems approximate the motor field flux by measuring the field current and then applying that current measurement to a pre-determined field current vs. flux curve. However, if the flux parameter that is approximated by the control system does not match the actual motor field flux, motor velocity oscillations occur. This mismatch between calculated and actual flux can arise in a conventional control system because the field current to flux curve is unique for each motor, varying according to time, motor size, and manufacturer. These unwanted motor velocity oscillations often occur under light motor loads when it is desirable to operate the motor under a weakened field. A field is weakened to conserve energy, prolong motor life, and operate above base speed. If the motor cannot maintain a consistent velocity under weakened field conditions, the end user will not be able to take advantage of the field weakening features.
What is needed, then, is a device and method that provides a more stable control of the speed of a DC motor by closely matching a calculated motor flux to the actual field flux in the motor.